. Student's manual of electro-therapeutics, embodying lectures delivered in the course on therapeutics at the Woman's medical college of the New York infirmary. vot. Tha dialwhich the point of this needle traverses is gen-erally divided into 360 equal degrees. Such agalvanometer is worse than useless, because itin no way tells the strength of the current pass-ing. The reason is this. While thirty cells ofa battery may deflect the needle of an ordinarycompass 2 5°, it by no means follows that sixtysimilar cells will deflect the same needle 5o°.The first few degrees of deflection are causedby a
. Student's manual of electro-therapeutics, embodying lectures delivered in the course on therapeutics at the Woman's medical college of the New York infirmary. vot. Tha dialwhich the point of this needle traverses is gen-erally divided into 360 equal degrees. Such agalvanometer is worse than useless, because itin no way tells the strength of the current pass-ing. The reason is this. While thirty cells ofa battery may deflect the needle of an ordinarycompass 2 5°, it by no means follows that sixtysimilar cells will deflect the same needle 5o°.The first few degrees of deflection are causedby a very weak current; the more the needledeflects from its normal position, the harder itis to move it, and the slower it , while the first cell may cause a devia-tion of 10° or 12°, each additional cell movesthe needle less and less. It follows that a graduation having degreesof equal length all the way round can be of no 14 ELECTRO- THERAPE UTICS. scientific use. The graduation should show thestrength of current passing, and hence the de-grees should be at first coarse, becoming, oneither side of the needle, progressively finer The best graduation is that into tenths ofmillivebers—i. e., a galvanometer where theneedle, when a given resistance is in circuit, bypointing at a certain number on the dial, shows. GALVANISM. 15 that a current so many millivebers strong ispassing. A galvanometer should be graduatedby passing, through a known resistance, the?current of a battery, the cells of which have anequal and known electro-motive force. In this way one is enabled to calculate exactlywhen one, two, or three, etc., millivebers arepassing, and to mark them on the dial. The best way to measure electro-motiveforce is as follows : Take a cell of known elec-tro-motive force and connect it with a galva-nometer, and note exactly the amount of deflec-tion that occurs. Then connect the cell or cellsto be tested with the galvanometer, and againnote the amount of
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